1. Field of the Invention
The present invention relates to a method of synthesizing a diamond film by using a chemical vapor deposition.
2. Description of the Background Art
Chemical vapor deposited (CVD) diamond films usually exhibit considerably high stress level. This has been considered due to their deposition conditions of using non-diamond substrates at high temperatures of 700.degree. C.-1,000.degree. C. and/or incorporating defects with microstructural changes during deposition. Under a high stress level, diamond films may crack or fracture during or after deposition. Also, in the case of synthesizing thick films on a large area of several-inch scale, stress may bend the diamond films.
Internal stress in films can be classified into two categories, thermal and intrinsic stresses. Thermal stress develops due to the difference in the thermal expansion coefficients between the diamond film and the substrate material. It appears only in the case that the diamond films are attached on the substrate, and can be calculated by the following equation (1), ##EQU1##
where, ##EQU2## PA1 indicates a biaxial Young's modulus of the diamond, .alpha..sub.f, and .alpha..sub.s are thermal expansion coefficient(TEC) of the diamond film and substrate, respectively, and T.sub.d and T.sub.m are the deposition and room temperature, respectively. At temperatures lower than the deposition temperature, thermal stress is "compressive(-)" if the TEC of the substrate is larger than that of the diamond. At room temperature, the magnitude of the thermal stress in the diamond films on refractory metals is reported to be several GPa.
On the other hand, in free standing film deposition, that is, films separated from the substrate, thermal stress is released at the instant when a film is separated from the substrate during the cooling down from the deposition temperature to room temperature after deposition. In this stage, the diamond film can crack or fracture, if temperature on the whole substrate is not uniform or adhesive force between the diamond film and the substrate is too large. This type of crack or fracture is called "thermal crack" or "thermal fracture". These thermal crack or thermal fracture can be suppressed by keeping the substrate temperature uniform or controlling the adhesive force between the diamond film and the substrate.
On the other hand, intrinsic stress develops during the growth and it is expected to be induced by the variation of microstructure and incorporation of defects in the diamond films. However its detail mechanism is still under debate. Intrinsic stress is generally accepted as "tensile stress(+)" ranging from several hundreds MPa to several GPa, although several researchers obtained "compressive stress". As well known in the field of hard coating like diamond deposition, such tensile stress was reported to cause cracks.
In the deposition of free-standing diamond films thicker than several hundreds .mu.m on a large area of several-inch scale, such tensile stress may cause serious problems. With increasing film thickness, tensile stress increases, while the strength of the diamond films decreases. This may crack the diamond films during deposition. This type of crack is called "growth crack". Also, a distribution of the tensile stress may bend the diamond films. The bending and the growth cracks of the diamond films make it impossible for the diamond film to be used as a material for an IR window, a microwave window, or a multi-chip module substrate, and increases its cost in application to any tools.
Accordingly, in order to resolve such problems, various researches have been done, and resultantly, various solutions have been proposed.
U.S. Pat. No. 5,270,077 discloses a use of an upwardly-convex substrate in order to compensate the bending phenomenon after observing that the diamond film bends concavely to its growth direction.
While, U.S. Pat. No. 5,587,124 discloses a use of an upwardly-concave substrate to synthesize an even diamond film after observing that the diamond film is bent in the opposite direction as opposed to U.S. Pat. No. 5,270,077.
These methods are effective; nevertheless, since the curvature is not easy to accurately estimate in machining the substrate, and in the case of using a curved substrate, a plasma intensity is to be varied depending on the position of the substrate, causing deterioration in uniformity of the diamond film.
U.S. Pat. No. 5,507,987 proposed a technique for reducing the bending of the diamond film by using a two-step synthesizing method, in which a diamond film having a predetermined thickness (approximately 150 .mu.m) is synthesized in a first step at a low growth rate at which the diamond film does not bend much, and then is synthesized in a second step at a high growth rate at which the diamond film bends substantially, thereby minimizing the bend of the diamond film. However, this method deteriorates the efficiency in synthesizing the film, and the bending of the diamond film is not completely overcome.
U.S. Pat. No. 5,587,013 also uses a two-step synthesizing method similar to that of U.S. Pat. No. 5,507,987, in which a condition for forming a concave film (deposition temperature: 880.about.950.degree. C., methane concentration: 2.5%CH.sub.4.about.3.5%CH.sub.4) and a condition for forming a convex film (deposition temperature: 800.about.850.degree. C., methane concentration: 0.5%CH.sub.4.about.1.5%CH.sub.4) are respectively observed and then the two conditions are combined to deposit an even diamond film. However, this method is also disadvantageous in that the efficiency for synthesizing the diamond film is degraded because the methane composition and deposition temperature are varied.